Can limit the degree of parallelism chosen by the optimizer for GROUP BY operations.
0 – 4294967295
3000000
Can be set temporary, for an individual connection, or for the PUBLIC group. Takes effect immediately.
When the PARALLEL_GBH_ENABLED option is ON, the value of PARALLEL_GBH_MIN_ROWS_PER_UNIT can indirectly limit the degree of parallelism chosen within the optimizer for GROUP BY operations by requiring that each unit of work to be done in parallel must have at least this many rows. The default of 3 million rows means that a table must have at least 6 million rows before the optimizer chooses to execute GROUP BY in parallel over that table.
The default of 3 million is appropriate for large databases on systems with numerous CPUs. For smaller systems or for servers where GROUP BY operations frequently involve more complex aggregates and grouping expressions, performance of some queries can be improved by setting this option to a lower value, such as 500,000.
